1. Field of the Invention
The present invention relates to full field digital mammography, in particular, to an apparatus for X-ray imaging a selected region of a patient's body by X-ray slot-scanning across the region and constructing a composite X-ray image of the region based on the scan. The invention has particular application to the field of mammography.
2. Description of the Related Art
Breast X-ray imaging is considered the most demanding of medical imaging procedures. With regard to spatial resolution, breast imaging specialists are now commonly interested in imaging lesions or masses that may require an imaging aperture that is about 50 microns in size or less. Contrast requirements are also demanding because lesions or masses to be visualized sometimes have x-ray absorption characteristics similar to that of the surrounding tissue. In this regard, 12 bit Analog to Digital Converters, which produce a contrast resolution of about 4000 distinguishable shades between black and white in the resulting image, is often found in many systems. 14-bit ADCs are also common today, and the market is moving to 16-bit ADCs, corresponding to about 65,000 shades of gray.
X-ray mammography is the most sensitive breast imaging modality currently available and is widely used in detecting and diagnosing the nature of small non-palpable breast lesions. Both film-based and digital X-ray systems are currently available for breast imaging. In film-based systems, x-rays are transmitted through the patient's breast and impinge upon a phosphor screen. Light emitted from the phosphor screen as a result of the absorption of x-rays is detected by light sensitive film. The film is then developed to yield an image of the patient's breast which can be viewed on a light box (negatoscope). In digital X-ray systems, a digital X-ray receiver is used on place of the film. The receiver yields an electric signal which can be digitally processed for viewing on a high resolution monitor. Until recently, only limited field of view digital systems, e.g., 5 cm by 5 cm field of view systems, were available, approximating film based systems in mammographic performance. More recently, full-field mammography systems have become available, though at considerable cost mostly due to the high cost of high resolution digital x-ray receivers based on flat panel X-ray detectors.
Heretofore, film-based systems have been most commonly used for breast imaging and improvements over the years in film-based, x-ray imaging technology have resulted in improved imaging capability and reduced radiation dosage. Film based systems are, however, subject to certain limitations. For example, film granularity and random film screen noise limits the spatial resolution of the resulting image. Furthermore, the time required to develop film images renders film-based systems less desirable for some applications, particular where, today, many patients (and physicians) wish to see the results of the diagnostic process immediately.
Recently, researchers have recognized that digital X-ray imaging systems offer potential advantages over film-based imaging systems. Digital mammography systems allow for substantially real-time imaging as may be desired. The stored digital imaging data can also be downloaded for transmission within a computer network and retrieved at remote workstations thereby facilitating information storage, consultation and computer image analysis.
Conventional digital mammography systems based on flat panel receiver have a number of disadvantages. Cost is one major factor—a large-format mammographic X-ray receiver is very expensive and sometimes very sensitive to careless handling and environmental conditions, driving the cost of the overall device and its maintenance costs upwards. Mammography X-ray imaging scanners that use linear matrix (narrow relative to imaging area width) detectors are known (which avoids the use of a very expensive full-field X-ray receiver), which utilize a pendulum-like structure, on which the X-ray detector is mounted, such as described in U.S. Pat. No. 5,526,394. However, such systems suffer from problems associated with flexing and mechanical deformations of the pendulum structure when the X-ray detector base on CCD is moving, which in turn causes a degradation of the image quality. Accordingly, there is a need in the art for a mammography scanner which is low in cost, and provides a high quality digital X-ray image.